1. Field of the Invention
This invention relates to a construction of a nozzle of plasma cutting torch.
2. Description of the Prior Art
A prior art plasma cutting torch of the kind specified above is described with reference to FIG. 1. The plasma cutting torch shown in FIG. 1 is constituted by a power supply system 16 which is comprised of a direct-current power supply 7, an electrode 1 connected by a connection with a negative pole of the d.c. power supply 7, a high frequency circuit 8 including a coupling coil 9 connected with the above-mentioned connection, a nozzle 2 connected by a connection in parallel with a positive pole of the d.c. power supply (This circuit includes a resistance 12 and an opening/closing switch 11 connected in series), a workpiece 6, and a bypass circuit including a bypass capacitor 10 for bypassing the d.c. power supply; a working gas supply system 17 for supplying a working gas from a gas supply source 13 through a gas pressure regulator 14 and a solenoid valve 15 into the nozzle 2; and a torch body system 18 which is comprised of the electrode 1, the nozzle 2 surrounding the electrode 1 and a gas passage 19 defined between the electrode 1 and the nozzle 2. (Although not shown in the drawing, in case the nozzle is of a water-cooled type, a water cooling unit is provided.) How to generate plasma arc in such a configuration will be described below. In the first place, the pressure of a gas supplied from a gas supply source 13 is regulated by the gas pressure regulator 14 connected thereto at a predetermined pressure P1. Thereafter, the solenoid valve 15 is opened and supply of the working gas into the nozzle 2 is commenced.
When the d.c. power supply is switched on, a voltage is applied between the electrode 1 (cathode) and the nozzle (anode), and a different voltage is applied between the electrode (cathode) and the workpiece 6 (anode). When a high frequency voltage is applied subsequently between the electrode 1 and the nozzle 2 through the high frequency circuit 8, a high frequency discharge will occur there-between, thus causing a dielectric breakdown in the space between the tip of the electrode 1 and the tip of the nozzle 2, thus causing a d.c. arc, i.e. a pilot arc between the electrode 1 and the workpiece 6. When conduction between the electrode 1 and the workpiece 6 is secured by the pilot arc, the opening/closing switch 11 is opened so as to generate a plasma arc between the electrode 1 and the workpiece 6, thus commencing cutting of the workpiece 6. Stating in brief, the plasma cutting is meant by fusion cutting of the workpiece 6 using the energy produced when the working gas is over-heated to a high temperature by drawing the arc discharge between the electrode 1 and the workpiece 6 thinly through a nozzle orifice 4.
The prior art plasma cutting torch having the above-mentioned configuration has, however, the following disadvantage.
Since the prior art plasma cutting torch is arranged such that, as mentioned above, the workpiece 6 is subjected to fusion cutting using the energy produced when the working gas is overheated to a high temperature by drawing the arc discharge between the electrode 1 and the workpiece 6 thinly through the nozzle orifice 4, as the discharge electric current is increased, the energy of the plasma arc will increase, thereby increasing the cutting capacity. In this case, however, there is a problem in that with increase in the value of discharge electric current an abnormal discharge referred to as "double arc" will occur and bring forth a disadvantage such as damage of the plasma nozzle (stating accurately, the nozzle of the plasma nozzle. To cope with this difficulty, when the prior art plasma cutting torch is used, it is necessary to control the discharge current so as to keep it less than a critical electric current at which double arc occurs.
(1) Point at issue in controlling critical electric current by increasing the gas supply pressure: PA0 (2) Point at issue in controlling critical electric current by changing over the gas supply pressure:
As can be seen from FIG. 3, since the critical electric current at which double arc occurs is raised by increasing the working gas supply pressure, the increase in the working gas supply pressure provides an advantage in that the resultant increase in the discharge electric current will improve the cutting capacity, whereas there is a disadvantage in that, as shown in FIG. 4, as the working gas supply pressure is increased the dielectric breakdown voltage at the time of application of a high frequency voltage for striking arc is also increased thus making it difficult to cause the dielectric breakdown. In other words, this is a disadvantage that it becomes difficult to generate arc. (Further, even if the dielectric breakdown voltage is raised, the generation of breakdown can be promoted by increasing the level of the high frequency output to be applied between the electrode and the nozzle, whereas the increase in the level of the high frequency output brings forth not only a disadvantage that the dielectric strength of every portion of the power supply must be improved, but also another difficulty that as the high frequency output increases the electrical noise generated by the plasma cutting torch will also increase.)
As an alternative to the above-mentioned method of controlling the critical electric current by increasing the working gas supply pressure, there is another method of controlling the critical electric current by changing over the working gas supply pressure. This method of controlling the discharge electric current comprises the steps of lowering the working gas supply pressure by means of the gas pressure regulator 14 when striking arc, and raising the gas supply pressure after striking of arc has occurred. This controlling method requires regulation of the gas pressure regulator 14 to be made each time arc is struck or suppressed, and is therefore disadvantageous in that striking of arc cannot be effected smoothly.
FIG. 2 is an enlarged view of the nozzle portions of the above-mentioned prior art plasma cutting torch. As shown in FIG. 2, the nozzle portion of the plasma cutting torch is comprised of an electrode 1 and a nozzle 2 disposed so as to define a working gas passage 19 which surrounds the electrode 1, and the nozzle 2 has a cylindrical nozzle orifice 4 formed through its leading end portion 4a.
In this prior art nozzle portion, in case a pilot arc 20 is generated when starting the plasma cutting torch, an anodic point is formed on the surface of the inner wall of the nozzle 2. Until a predetermined migrating electric current flows, this anodic point is moved by the flow of the working gas from the surface of the inner wall of the nozzle 2 to the leading end portion 4a of the nozzle orifice 4 of the nozzle 2 and held there for some time. In this condition, the leading end portion 4a of the nozzle orifice 4 having the anodic point formed thereon is worn away and deteriorated thus deteriorating the quality of the workpiece to be cut.